A developable latent image is formed in a silver halide emulsion layer of a radiographic element when it is imagewise exposed to X-radiation. However, much of the highly energetic X-radiation simply passes through the radiographic element without being absorbed. The useful native sensitivity (i.e., maximum absorption capability) of silver halide emulsions lies in the near ultraviolet (300-400 nm) and blue (400-500 nm) portions of the spectrum. The native sensitivity of silver chloride is negligible beyond 450 nm, with sensitivity dropping approximately 2 orders of magnitude between 380 and 420 nm. The native sensitivity of silver bromide is negligible beyond 500 nm, with sensitivity dropping approximately 2 orders of magnitude between 450 and 490 nm. The native sensitivity of silver bromoiodide (3 mole % iodide) is negligible beyond 550 nm, with sensitivity dropping approximately 2 orders of magnitude between 470 and 530 nm. Thus, not only do silver halides fail to absorb efficiently in the green portion of the spectrum, the absorption of silver halides in the longer wavelength regions of the blue spectrum are relatively limited.
It is, of course, known that the spectral response of silver halide emulsions can be extended into the green and red portions of the spectrum by adsorbing one or more spectral sensitizing dyes to the surfaces of the silver halide grains in the emulsions. Although routine, spectral sensitization is not without its disadvantages. The dyes themselves are complex organic molecules that, on a weight basis, are more expensive that silver, but, unlike silver, are not recoverable for reuse. Further, emulsion addenda that also adsorb to grain surfaces, such as antifoggants and stabilizers, can displace the dyes, leading to reduced spectral sensitivity.
To reduce patient exposure to X-radiation it is conventional practice in medical radiology to employ silver halid in combination with intensifying screens, where the intensifying screen contains a phosphor layer that absorbs X-radiation more efficiently than silver halide and emits longer wavelength electromagnetic radiation which silver halide can more efficiently absorb. Blue emitting intensifying screens capable of imagewise exposing silver halide radiographic elements within the spectral region of native grain sensitivity are known in the art. Although many blue emitting phosphors are known, calcium tungstate has for many years been the standard blue emitting phosphor for use in intensifying screens against which blue emitting intensifying screens have been compared.
Kroger et al U.S. Pat. No. 2,542,336 discloses phosphors containing titanium as an activator and having a matrix comprised of one or more of the oxides of zirconium, hafnium, thorium, germanium or tin to which may be added either acid oxides or basic oxides or both. Disclosed basic oxides are the oxides of sodium, potassium, rubidium, cesium, lithium, barium, calcium, strontium, magnesium, beryllium and zinc. Disclosed acid oxides are SO.sub.3, B.sub.2 O.sub.3, P.sub.2 O.sub.5 and SiO.sub.2. Titanium activated zirconium oxide, magnesium stannate, calcium zirconate and zirconium phosphate are each specifically disclosed.
Titanium activated germanium oxide is a blue emitting phosphor, but investigations, undertaken in connection with this invention and included among comparative examples below, have revealed titanium activated germanium oxide to exhibit low emission intensities.
Titanium activated hafnium oxide exhibits peak emission in the longer wavelength (approx. 475 nm) blue portion of the spectrum, with a substantial portion of its total emission extending into the green region of the spectrum. L. H. Brixner, "Structural and Luminescent Properties of the Ln.sub.2 Hf.sub.2 O.sub.7 -type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984, after reporting the properties of Ti.sup.+4 as an activator for rare earth hafnates, noted a high level of performance for titanium activated optical grade hafnia (HfO.sub.2), but considered the phosphor impractical for intensifying screen use based on the price of optical grade hafnia. Optical grade hafnia contains less than 3.times.10.sup.-4 mole of zirconia (ZrO.sub.2) per mole of hafnia.
Bryan et al U.S. Pat. No. 4,988,880 discloses that efficient X-ray intensifying screens can be constructed from titanium activated hafnia phosphors containing minor amounts of zirconium, but higher amounts than found in optical grade hafnia, specifically: EQU Hf.sub.1-z Zr.sub.z
where
z ranges from 4.times.10.sup.-4 to 0.3. Sharp losses in emission intensities were found at higher values of z.
Phosphors which contain germanium, zirconium or hafnium and oxygen with oxygen being complexed with other nonmetals, such as sulfur, boron, phosphorus, silicon and the like, produce distinctly different crystal structures than those of hafnium and/or zirconium germanate and are not considered relevant to this invention.
Bryan et al U.S. Pat. No. 4,988,881 discloses the preparation of lithium hafnate phosphors. In the preparation of the lithium hafnate phosphor hafnia has been also formed as a secondary phase.